1. Technical Field
This invention relates to engine control systems and more particularly to methods and systems for controlling EGR rate in diesel engines.
2. Background
As is known in the art, diesel engines provide great fuel economy benefits compared to stoichiometric spark ignited engines (e.g., gasoline internal combustion engines). As is also known in the art, it is desirable to reduce emissions from both types of such engines. One such emission to be reduced is NOx (oxides of nitrogen). One technique used to reduce such NOx emission is Exhaust Gas Recirculation (EGR). EGR operates by recirculating engine exhaust back to the engine""s intake manifold. EGR uses an EGR valve disposed in a duct between the engine exhaust manifold and the engine intake manifold. To enable a flow of exhaust to pass from the exhaust manifold and the intake manifold through the EGR valve, a differential pressure must exist across the EGR valve. The intake throttle has the effect of creating a pressure in the intake manifold that is lower than the pressure in the exhaust manifold thereby providing the requisite differential pressure across the EGR valve.
With a diesel engine, the power developed by the engine is typically controlled by controlling the amount of fuel injected into the engine cylinders rather than through the use of a throttle at the intake of the engine. Thus, while it is desired to use EGR to reduce NOx in a diesel engine, the absence of a throttle may result in insufficient differential pressure across the EGR valve to obtain adequate EGR rates for required NOx reduction. Thus, with a diesel engine, while there may be the absence of a throttle for control of engine power, a throttle is sometimes placed in the path of the engine intake to obtain a differential pressure (and hence exhaust recirculation flow) across the EGR valve. Such technique has provided EGR rates of up to 60% of the in-cylinder flow through the EGR valve. One technique used to control the intake throttle of a diesel engine is to provide a nominal setpoint for the throttle. This setpoint is established as a function of engine speed, fuel quantity, engine temperature, ambient pressure and temperature. The EGR rate is controlled in closed loop fashion by regulating the EGR valve to achieve a setpoint established for a measured mass air flow through the throttle. A disadvantage of such technique is that, in general, the EGR valve is not as open as it could be, the pressure drop from the exhaust to the intake manifold is larger than it has to be, and pumping losses against the throttle are higher than necessary. The overall result is therefore a lower than optimal fuel economy.
In accordance with the present invention, a method is provided for controlling the EGR rate of an internal combustion engine. The engine has an intake throttle for controlling mass flow to an intake of the engine and an EGR valve for controlling flow from the exhaust of the engine back to the intake of the engine along with air passing through the intake throttle. The method includes measuring the mass air flow passing to the intake throttle. The measured mass air flow is compared with a desired mass air flow. An error signal is produced representative of a difference between the measured mass air flow and the desired mass air flow. A pair of control signals is produced in response to such produced error signal. One of the pair of control signals is used to adjust the intake throttle to control mass air flow through such intake throttle. The other one of the pair of control signals is used to adjust EGR rate through the EGR valve. The pair of control signals operate the intake throttle and the EGR valve to drive the error signal towards a null.
In one embodiment, one of the control signals used to adjust the EGR rate is used to provide such adjustment only when the intake throttle is in a position to provide substantially maximum mass air flow through such intake throttle to the intake of the engine.
In one embodiment the pair of control signals operate to drive the throttle to a closed position only when such error signal is unable to be driven towards the null solely from adjustment by the EGR valve.
In one embodiment, the engine includes a turbocharger having a compressor and a turbine coupled to the compressor. A portion of engine exhaust is directed to the EGR valve and another portion of the engine exhaust is directed to the turbine. The one of the control signals adjusting the EGR valve adjusts such portions. The portion of the engine exhaust to the turbine drives the compressor that in turn drives the mass air flow. The mass air flow from the compressor passes to the intake throttle and the intake throttle adjustment adjusts the mass air flow.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.